HABER PROCESS

What is ammonia? It is made industrially by reacting nitrogen with hydrogen in the Haber process. It is a reversible reaction, so it never goes to completion. hydrogennitrogen + ammonia N 2 (g)3H 2 (g)2NH 3 (g) + Ammonia is an important compound in the manufacture of fertilizer and other chemicals such as cleaning fluids and floor waxes. Why is this a problem for companies making ammonia?

The Haber process

A flow diagram Purifier and drier N2N2 H2H2 Compression Chamber (200 atm) Clean Dry N 2 & H 2 Heat exchanger Catalytic chamber Condenser Liquid NH 3 to storage Unreacted N 2, H 2

What is yield? The amount of product made in a reaction is called the yield and is usually expressed as a percentage. ammonia yield (%) pressure (atm) The yield of ammonia produced by the Haber process depends on the temperature and pressure of the reaction.

What is the Haber compromise? In practice, though, these conditions are not used. Why? The highest yield of ammonia is theoretically produced by using a low temperature and a high pressure. A compromise is reached to make an acceptable yield in a reasonable timeframe while keeping costs down. Lowering the temperature slows down the rate of reaction. This means it takes longer for ammonia to be produced. Increasing the pressure means stronger, more expensive equipment is needed. This increases the cost of producing the ammonia.

Temperature, pressure and yield

N 2 (g) + 3H 2 (g) 2NH 3 (g) :  = - 92 kJ mol -1 ConditionsPressure20000 kPa (200 atmospheres) Temperature °C Catalystiron HABER PROCESS

N 2 (g) + 3H 2 (g) 2NH 3 (g) :  = - 92 kJ mol -1 ConditionsPressure20000 kPa (200 atmospheres) Temperature °C Catalystiron Equilibrium theory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules HABER PROCESS

N 2 (g) + 3H 2 (g) 2NH 3 (g) :  = - 92 kJ mol -1 ConditionsPressure20000 kPa (200 atmospheres) Temperature °C Catalystiron Equilibrium theory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetic theory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy HABER PROCESS

N 2 (g) + 3H 2 (g) 2NH 3 (g) :  = - 92 kJ mol -1 ConditionsPressure20000 kPa (200 atmospheres) Temperature °C Catalystiron Equilibrium theory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetic theory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy Compromise conditions Which is better? A low yield in a shorter time or a high yield over a longer period. The conditions used are a compromise with the catalyst enabling the rate to be kept up, even at a lower temperature. HABER PROCESS

IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS MAKING FERTILISERS80% of the ammonia produced goes to make fertilisers such as ammonium nitrate (NITRAM) and ammonium sulphate NH 3 + HNO 3 ——> NH 4 NO 3 2NH 3 + H 2 SO 4 ——> (NH 4 ) 2 SO 4 MAKING NITRIC ACIDammonia can be oxidised to nitric acid nitric acid is used to manufacture... fertilisers (ammonium nitrate) explosives (TNT) polyamide polymers (NYLON) HABER PROCESS

The Haber compromise To produce a high yield of ammonia, but with a fast rate of reaction and without the need for overly expensive equipment, the Haber process is carried out at 450 °C and 200 atmospheres. The most important factor in deciding what conditions to use is therefore not yield, but total cost. raw materials equipment energy wages What costs are involved in the industrial production of ammonia?

Maximizing productivity What else can be done to maximise productivity in the manufacture of ammonia? An iron catalyst is used to increase the rate of reaction. It speeds up both the forward and backward reaction, so the position of equilibrium is not affected. The ammonia is cooled, liquefied and then removed as it is produced. This causes the equilibrium to shift to the right to produce more ammonia. Unreacted nitrogen and hydrogen are recycled and given another chance to react.

Stages of the Haber process

Glossary closed system – A system in which reactants and products cannot be added or removed once the reaction has begun. dynamic – An equilibrium in which the forward and backward reactions take place at the same rate, so no overall change takes place. Haber process – The industrial-scale process for making ammonia from nitrogen and hydrogen. irreversible – A reaction that is impossible or very difficult to reverse. reversible – A reaction in which the product(s) can be turned back into the reactants. yield – The amount of product obtained from a reaction, usually expressed as a percentage.